Method for producing active or passive components on a polymer basis for integrated optical devices

ABSTRACT

The object of the process according to the present invention is to fabricate active and passive optoelectronic components of a high quality, and having a high level of integration and high packing density.  
     According to the present invention, a patternable polymer resist layer of a high quality is deposited onto an optoelectronic component. An etching mask is used in conjunction with a high-grade anisotropic deep etching to produce a pattern which is filled with monomers through gas-phase or liquid-phase diffusion. The optical properties of the optical component can be selectively changed as a function of the type of monomers used for the diffusion, as well as of the temperature and application time.  
     The process according to the present invention makes it possible to increase the packing density of future integrated monomode optics and simultaneously produce large quantities in a cost-effective manner.

[0001] The present invention relates to the fabrication of active andpassive polymer-based, optoelectronic components. The technical task athand is to devise a method directed to the fabrication of passive andactive optoelectronic components having a high level of integration andhigh packing density. The fabrication process should make it possible toinfluence the parameters and properties of the optoelectronic componentto be produced, in particular, to selectively influence the refractiveindex, nonlinear optical property, polarizability, double refraction andamplification properties during the fabrication process.

[0002] As described in

[0003] 1.] R. Kashyap, in “Photosensitive Optical Fibers: Devices andApplications”, Opt. Fibres Techn. 1, pp. 17-314 (1994), present-dayfabrication processes for components and circuits of integrated opticsare based on optical fiber technology which strives for an “all-fiber”solution for the circuits required in telecommunications. Integratedoptical waveguide circuits are constructed, together with active andpassive components on expensive semiconductor substrates, using evenmore expensive molecular beam epitaxy or metal organic deposition fromthe vapor phase, to implement the optical circuits required intelecommunications. A description of such processes can be found in thefollowing sources:

[0004] 2.] C. Cremer, H. Heise, R. März, M. Schienle, G. Schulte-Roth,H. Unzeitig, “Bragg Gratings on InGaAsP/InP-Waveguides as PolarizationIndependent Optical Filters” J. of Lightwave Techn., 7, 11, 1641 (1989);

[0005] 3.] R. C. Alferness, L. L. Bühl, U. Koren, B. I. Miller, M. G.Young, T. L. Koch, C. A. Burrus, G. Raybon, “Broadly tunable InGaAsP/InPburied rib waveguide vertical coupler filter”, Appl. Phys. Lett., 60, 8,980 (1992);

[0006] 4.] Wu, C. Rolland, F. Sheperd, C. Larocque, N. Puetz, K. D.Chik, J. M. Xu, “InGaAsP/Inp Vertical Filter with Optimally DesignedWavelength Tunability”, IEEE Photonics Technol. Lett., 4, 4, 457 (1993);

[0007] 5.] Z. M. Chuang, L. A. Coldren “Enhanced wavelength tuning ingrating assisted codirectional coupler filter”, IEEE PhotonicsTechnology Lett., 5, 10, 1219 (1993).

[0008] Also known is a process for fabricating waveguide circuits frompolymeric waveguides using mask-assisted exposure processes, asdescribed in source 6.] by L-H. Lösch, P. Kersten and W. Wischmann in“Optical Waveguide Materials” (M. M. Broer, G. H. Sigel Jr., R. Th.Kersten, H. Kawazoe ed) Mat. Res. Soc. 244, Pittsburg, Pa. 1992, pp.253-262.

[0009] A further known design approach is based on defining thewaveguides by etching a step into optically thinner layers. A process ofthis kind is described by 7.] K. J. Ebeling in “IntegrierteOptoelektronik” (Springer Verlag 1989) 81.

[0010] 1. A further known process is based on silylation. In thesilylation process, waveguides are already defined in NOVOLAK, andchecked for their usability in integrated optics, as described in source8.] by T. Kerber, H. W. P. Koops in “Surface imaging with HMCTS on SALresists, a dry developable electron beam process with high sensitivityand good resolution”, Microelectronic Engineering 21 ((1993) 275-278.

[0011] 2. The processes required for this and for accurate processcontrol are described in source 9.] by H. W. P. Koops, B. Fischer, T.Kerber, in “Endpoint detection for silylation processes with waveguidemodes”, Microelectronic Engineering 21 (1993) 235-238, and in source10.] by J. Vac, SCI Technol. B 6 (1) (1988) 477.

[0012] Substantial differences in refractive indices can be produced byimplanting ions at high energies and high doses in PMMA. Processes ofthis kind are described in source 11.] by R. Kallweit, J. P-Biersack in“Ion Beam Induced Changes of the Refractive Index of PMMA”, RadiationEffects and Defects in Solids, 1991, vol. 116, pp. 29-36, and in source12.] by R. Kallweit, U. Roll, J. Kuppe, H. Strack “Long-Term Studies onthe Optical Performance of Ion Implanted PMMA Under the Influence ofDifferent Media”, Mat. Res. Soc. Symp. Proc. Vol. 338 (1994) 619-624. Inthis context, differences in the refractive indices in solid PMMAmaterial of up to 20% are obtained. However, masking processes must beused for patterning. Due to the high ion energy and the requiredabsorber layer thickness in the mask, the resolution is limited by theedge roughness that is attainable using mask fabrication technologies.Electrically switchable regions incorporated in waveguides can beproduced by diffusing poled, nonlinearly optical materials in polymers.In this manner, one can achieve a link to electrical adjustability ofoptical paths, or to the influencing of optical processes.

[0013] 13.] M. Eich, H. Looser, D. Y. Yoon, R. Twieg, G. C. Bjorklund,“Second harmonic generation in poled organic monomeric glasses”, J. Opt.Soc. Am. B, 6, 8, (1989);

[0014] 14.] M. Eich, A. Sen, H. Looser, G. C. Björklund, J. D. Swalen,R. Twieg, D. Y. Yoon, “Corona Poling and Real Time Second HarmonicGeneration Study of a Novel Covalently Functionalized AmorphousNonlinear Optical Polymer”, J. Appl. Phys., 66, 6, (1989)R. Birenheide;

[0015] 15.] M. Eich, D. A. Jungbauer, O. Herrmann-Schönherr, K. Stoll,J. H. Wendorff, “Analysis of Reorientational Processes in LiquidCrystalline Side Chain Polymers Using Dielectric Relaxation,Electro-Optical Relaxation and Switching Studies”, Mol. Cryst. Liq.Cryst., 177, 13 (1989);

[0016] 16.] M. Eich, G. C. Björklond, D. Y. Yoon, “Poled AmorphousPolymers of Second Order Nonlinear Optics”, Polymers for AdvancedTechnologies, 1, 189 (1990) M. Stalder, P. Ehbets, “Electricallyswitchable diffractive optical element for image processing”, OpticsLetters 19, 1 (1994).

[0017] Free configurability of the pattern is achieved if, using the newprocess of additive lithography, three-dimensional patterns and periodicarrangements are constructed on any desired, inexpensive substrates, andif the refractive index of the deposited material is adapted to thetask, by properly selecting the precursor material, as well as thesources listed below, are named as sources for the aforementionedsubject area.

[0018] 17.] M. Stalder, P. Ehbets, “Electrically switchable diffractiveoptical element for image processing”, Optics Letters 19, 1 (1994);

[0019] 18.] H. W. P. Koops, R. Weiel, D. P. Kern, T. H. Baum, “HighResolution Electron Beam Induced Deposition”, Proc. 3 1. Int. Symp. OnElectron, Ion, and Photon Beams, J. Vac. Sci. Technol. B 6(1) (1988)477;

[0020] 19.] H. W. P. Koops, J. Kretz, M. Rudolph, M. Weber “Constructive3-dimensional Lithography with Electron Beam Induced Deposition forQuantum Effect Devices”, J. Vac. Sci. Technol. B 10(6) November,December (1993) 2386-2389;

[0021] 20.] H. W. P. Koops, J. Kretz, M. Rudolph, M. Weber, G. Dahm, K.L. Lee “Characterization and application of materials grown by electronbeam induced deposition”, Invited lecture Micro Process 1994, Jpn. J.Appl. Vol. 33 (1994) 7099-7107, part. 1 no. 12B, December 1994;

[0022] 21.] Hans W. P. Koops, Shawn-Yu Lin, “3-Dimensional PhotonCrystals Generated Using Additive Corpuscular-Beam-Lithography” patentspecification filed on Aug. 20, 1995.

[0023] It is, thus, possible to construct narrow-band, geometrical andpermanently adjustable filters and highly reflective mirrors on aminiaturized scale from photon crystals. If the photon crystals producedusing deposition techniques are combined with nonlinear, opticalmaterials in the interstices of the deposited materials, it is possibleto obtain miniaturized, adjustable optical components [source 21].Present-day surface-imaging processes make it possible, using opticalphase masks and steppers and, with the use of dry etching processes, toachieve the resolution and height ratios required for optical gratingsand other optical elements. This can be achieved using the lithographyand process equipment of the manufacturers of electronic storage deviceshaving a 1 G-bit size, and corresponding resolution. High-throughputproduction processes are used in corpuscular-beam opticalminiaturization techniques, as explained in the following sources:

[0024] 23.] H. Koops, 1974, German Patent 2446 789.8-33“Corpuscular-Beam Optical Device for Corpuscular Irradiation of aPreparation”;

[0025] 24.] H. Koops, 1974, German Patent 2460 716.7 “Corpuscular-BeamOptical Device for Corpuscular Irradiation of a Preparation”;

[0026] 25.] H. Koops, 1974, German Patent 2460 715.6 “Corpuscular-BeamOptical Device for Corpuscular Irradiation of a Preparation in the Formof a Two-Dimensional Pattern having a Plurality of IdenticalTwo-Dimensional Elements”;

[0027] 26.] H. Koops, 1975, German Patent 2515 550.4 “Corpuscular-BeamOptical Device for Imaging a Mask onto a Preparation to be Irradiated”;

[0028] 27.] H. W. P. Koops, “Capacities of Electron Beam Reducing ImageProjection Systems having Dynamically Compensated Field Aberrations”Microelectronic Engineering 9 (1989) 217-220.

[0029] A further known miniaturization technique is based on techniquesusing small mask templates as described in the following sources:

[0030] 28.] H. Elsner, P. Hahmann, G. Dahm, H. W. P. Koops “MultipleBeam-shaping Diaphragm for Efficient Exposure of Gratings” J. Vac. Sci.Technol. B 0(6) November, December (1993) 2373-2376;

[0031] 29.] H. Elsner, H.-J. Doring, H. Schacke, G. Dahm, H. W. P. Koops“Advanced Multiple Beam-shaping Diaphragm for Efficient Exposure”,Microelectronic Engineering 23 (1994) 85-88.

[0032] Miniaturization can also be achieved through the use ofelectron-beam-induced deposition in projectors.

[0033] 30.] M. Rüb, H. W. P. Koops, T. Tschudi “Electron-beam-induceddeposition in a reducing image projector”, Microelectronic Engineering 9(1989) 251-254.

[0034] At the present time, one does not know of integrated opticalpatterns, where the process of refractive index modulation is employedby diffusing nonlinear optical, high-refractive-index or liquid-crystalmonomers into existing polymers, in conjunction with free-standingpolymer patterns, where the refractive index difference with respect tothe vacuum is used as the essential step for the refractive indexincreases.

[0035] The process according to the present invention for fabricatingactive and passive optical components is based on the generally knownprocesses of surface imaging, to produce an oxygen-resistant etchingmask in unexposed regions, and of diffusing molecules into patternedpolymer layers.

[0036] According to the present invention, at least one patterned,highly sensitive polymer resist layer is deposited on an optoelectroniccomponent, made of glass and conductor path, or of substrate. Definedregions of the polymer resist layer are subsequently exposed, producingan etching mask. High-grade anisotropic deep etching is performed on theunprotected regions to transfer the etching mask to the polymer resistlayer located underneath the etching mask. The exposed regions of thepolymer resist layer are ablated in the vertical direction, uncoveringthe unexposed side surfaces of the regions protected by the etchingmask.

[0037] In the ensuing gas-phase or liquid-phase diffusion process, theunexposed polymer resist layer is filled with monomers, from itssurface, through the mask of the surface masking and, from its sidesurfaces uncovered by the oxygen deep etching, with the application ofheat. In the process, monomers are used, which are suited for breakingup the already existing pattern of the polymer, and for repatterning it,so that the optical properties of the optoelectronic component can beselectively changed as a function of the type of monomers used, and alsoas a function of the temperature and application time. In the diffusionprocess, the polymer then swells on all sides and, therefore, thepreviously lost edge region can be compensated selectively by theswollen material and, in a controlled manner, by the diffusion time andtemperature. In addition, because of the acting surface tension, thesurfaces produced by the swelling are very smooth, i.e. peak-to-valleyheights in the 2 nm range are obtained. The obtained refractive indexprofile is assured in the long term by UV hardening and by deepcrosslinking of the diffused molecules carried out after the diffusion.

[0038] By diffusing heavy metal oxide-containing, nonlinearly optical orliquid-crystal monomers or also molecules containing “rare earths” intothe uncovered, deep polymer patterns, it is possible, in addition topassive materials, also to produce nonlinearly optical, active materialsin selected regions. It is, therefore, possible to produce diffusedrefractive index profiles in regions defined by optical and corpuscularbeam lithography.

[0039] The following elucidates how the object of the present inventionis achieved, on the basis of an exemplary embodiment.

[0040]FIG. 1 illustrates the diagram for fabricating refractive indexprofile patterns, with the aid of chemical diffusion, in the expandedsilylation process.

[0041] A highly sensitive, patternable polymer layer is deposited on thesubstrate composed of glass and conductor path. In the exemplaryembodiment, Novolak is used. The etching mask is produced by theexposure of defined regions of the polymer resist layer, correspondingto the later component, in conjunction with a process of silylating theunexposed regions. By combining the silylation process used forhigh-resolution pattern definition with the dry etching of thecross-linked polymers to produce the great height-to-width ratios of thepatterns, it is ensured that the non-cross-linked/unexposed material isavailable for further chemical diffusion of monomers for the variousdesired effects. This part of the material is normally removed in thedevelopment process when the negative working novolak is exposed. It isretained after dry etching as a result of the silylation. If thesilylation process is begun with a short isotropic process attacking thesilicon oxide of the silylation mask, the pattern broadens, but therough edge structure of the silylated region, obtained by the shot noiseof the electron exposure in the edge region of the mask, is smoothed.

[0042] Thus, in the following anisotropic dry etching process, whichemploys an etchant that attacks the silicon oxide of the etching mask,it is possible to use directional oxygen ions to achieve smooth sidewalls of the polymer. This solves the shot-noise edge roughness probleminevitably encountered in corpuscular beam optics. The scattering lossesto be expected at the rough surfaces are also minimized.

[0043] In the subsequent diffusion process, the polymer then swells onall sides, with the result that the edge region that had been previouslylost can be compensated for in a controlled manner by the swollenmaterial, and by the diffusion time and temperature. By diffusing heavymetal oxide-containing, nonlinearly optical compounds or other similarcompounds, or molecules contained in “rare earths”, into the uncovereddeep polymer patterns, nonlinearly optical active materials can also beproduced, in addition to passive materials, in selected regions. Thus,diffused refractive index profiles can be produced in regions defined byoptical and corpuscular beam lithography. Such diffusion can take place,as in a conventional manner, into unetched polymer layers, resulting inrefractive index differences of up to 10%. If diffusion is carried outin polymer layers already patterned using wet chemical development or bydry etching, then refractive index differences of between 1.5 and 3 canbe achieved.

[0044] Using this process, one can increase the refractive indexdifference of 10-³ to 10-⁴ to 0.06, in the case of UV- andelectron-exposed plexiglass, as the difference in the refractive indexbetween silylated and unsilylated novolak. The refractive indexdifferences attained can be further increased by dissolving resistregions, negatively polymerized by the exposure process, out of theoptically active and passive pattern, using high-resolution oxygen dryetching, resulting in refractive index differences with respect tovacuum of n=1. In the case of the free-standing, silylated region, therefractive index difference increases to 1.57, while it is 1.63 for theunsilylated material. Consequently, the finished component is made ofchemically inert, saturated materials of glass-like composition and gooddurability. The diffused regions can be cross-linked with long-termstability by UV deep cross-linking, rendering possible a long servicelife for the components. Electrical and integrated optical componentscan be easily combined in the layers of the component, because theprocess involves processes that have been used for a number of years inlithography. Fabrication is accelerated, because the novolak resistsystems are characterized by approx. 20 times higher sensitivity incomparison with PMMA (plexiglass). The oxygen etching processadditionally tempers the regions diffused with chemicals and, thus,ensures the durability of the components.

[0045] The process according to the invention makes it possible toproduce high-quality and highly effective diffractive patterns havingfew grating planes or lines and, thus, to fabricate integrated opticalcomponents, such as couplers, gratings, selectors and reflectors havingfew grating periods. By employing such high refractive index differencesin the optical patterns and gratings, one can achieve the same opticalquality using much shorter components, than is possible usingpolymer-plexiglass techniques. This greatly increases the packingdensity of the integrated optical elements in miniaturized integratedoptics. The following are possibilities for implementing the opticalcomponents according to the invention on a large scale:

[0046] 1. Using beam-guiding or die-mask-projecting lithography toolsfeaturing variably shaped beams, fast development steps could be carriedout in the technology, in short time periods, for small quantities.

[0047] 2. The optoelectronic components according to the presentinvention can be preferably mass produced cost-effectively usingconventional lithography processes known from optical memory deviceconstruction, such as corpuscular-beam and optical-template projectiontechniques, and optical mask projection techniques, including X-raylithography processes. The process makes it possible to increase thepacking density of future integrated monomode optics and simultaneouslyproduce large quantities in a cost-effective manner.

What is claimed is:
 1. A process for fabricating active and passivepolymer-based components for use in integrated optics, incorporating theprinciple of gas-phase or liquid-phase diffusion, characterized in thatat least one highly sensitive, patternable polymer resist layer isdeposited onto one optoelectronic component; an etching mask is producedby exposing defined regions of the polymer resist layer; high-gradeanisotropic deep etching is performed on the unprotected regions totransfer the etching mask to the polymer resist layer located underneaththe etching mask, the exposed regions of the polymer resist layer beingablated in the vertical direction, uncovering the unexposed sidesurfaces of the regions protected by the etching mask; the unexposedpolymer resist layer is filled with monomers, from its surface, throughthe mask of the surface masking and, from its side surfaces uncovered bythe deep etching, through gas-phase or liquid-phase diffusion, with theapplication of heat, the monomers being suited for filling the alreadyexisting pattern of the polymer, for breaking it up and for repatterningit, it being possible to selectively change the optical properties ofthe optoelectronic component as a function of the type of monomers usedfor the doping, as well as of the temperature and application time. 2.The process according to claim 1, characterized in that the materialswelling inevitably occurring during the diffusion process isselectively controlled through the diffusion time and the processtemperature, until pattern inaccuracies have again been compensated, thesurface roughness caused by the effectiveness of the surface tension inthe material being simultaneously smoothed.
 3. The process according toclaim 1, characterized in that, vacuum or air is used at standardpressure in the interstices of the patterned polymer to adjust adifference in refractive indices of >1.5 with respect to the patterns inthe filled polymer, producing optical elements of extremely high qualityand having few periods and, thus, few refracting surfaces.
 4. Theprocess according to claim 1, characterized in that the polymer patternfilled with nonlinear material is surrounded by electrical electrodes,and in that the optical properties of the polymer pattern are influencedby controlling the electrical field applied between the electricalelectrodes.
 5. The process according to claim 1, characterized in thatthe polymer pattern filled with nonlinearly optical material isconnected to waveguides, through which light is injected into thepolymer pattern, and the optical properties of the polymer pattern areinfluenced by varying the injected light.
 6. The process according toclaim 1, characterized in that the etching mask is produced by exposingdefined regions of the polymer resist layer in conjunction withsilylation of the unexposed regions of the polymer resist layer and,subsequent to the silylation, the etching mask is smoothed at its edgesby an isotropic etching attack using an agent which attacks the siliconoxide of the etching mask.